In order to assess the possibility of isomerization during radical polymerization of vinyl mercaptobenzothiazole (VMBT), the isomerization of VMBT and its related compounds was investigated in the presence or absence of catalyst. In the absence of catalyst, VMBT was found to polymerize through participation of only its vinyl group at temperatures below 200°C. When VMBT was heated at 200°C in the presence of iodine as a catalyst, mercaptobenzothiazole (MBT) and polymer containing both a double bond and the benzothiazolinethione unit were obtained, but no isomerized N-vinyl monomer (VBTT) was produced. However, ethyl mercaptobenzothiazole (EMBT), a saturated model of VMBT, was easily isomerized to N-ethyl benzothiazolinethione under the same conditions. Therefore, it was concluded that VMBT was polymerized by the ordinary vinyl polymerization mechanism, even at 200°C without isomerization, but the MBT group in the resulting VMBT polymer underwent isomerization and elimination if isomerization catalyst was present.